Moisture detectors are used in a variety of applications using resistance or capacitance or a combination of the two. Resistance of a material is proportional to the amount of moisture in the material measured by inserting two probes in the material and measuring the resistance between the probes. Planar capacitor configurations can measure the change in capacitance due to the change in relative permittivity as a function of the moisture content in the material under test.
Advances in technology and in particular Radio-Frequency IDentification (RFID) for detecting moisture in material has significantly reduced the cost and size of moisture sensors. RFID tag incorporating a tag IC connected to an antenna attaches to the material under test. The reader activates the tag IC transmitting radio-frequency (RF) at the tag antenna and the tag circuit connected to the antenna converts the RF into a DC power for powering the IC and communicating back to the reader. The relative permittivity of the antenna changes with the changing moisture content which is communicated to the reader and used to signal moisture content.
RFID sensor tags do not require any batteries in order to operate, receiving power from the RF transmitted from a reader to power and signal a tag to respond. Reading distance is a function of the reader's transmitted power, tag antenna size, any electrical obstruction, and orientation of tag antennas relative to the reader transmitting antenna. Tag reading distance and reliability due to the issues mentioned, is a problem for many applications.